In the past year we developed an in vitro system for studying replication and transcription, using Sendai virus nucleocapsids extracted from infected cells. To determine the function of the polypeptide P associated with Sendai virus nucleocapsids, we analyzed the ability of nucleocapsids to synthesize RNA in vitro after removal of P following treatment with either salt plus detergent or Staphylococcus aureus protease. The salt plus detergent-treated nucleocapsids lost about 90% of their RNA synthetic ability; RNA synthesis by protease-treated nucleocapsid was reduced only 33%. These findings suggest that the loss of RNA synthesizing activity following salt plus detergent treatment is not due to the removal of polypeptide P. The retention of RNA synthetic activity by the protease-treated nucleocapsids suggests that more than 90% of the P polypeptides found on nucleocapsids are not necessary for RNA synthesis.